It is well-known in the prior art to design filter structures comprising an air dielectric through-line with resonators coupled at quarter-wavelength spacings. One such structure is described in U.S. Pat. No. 4,262,269 to Griffin et al. This U.S. patent discloses a five-pole notch filter having five quality factor (Q) enhanced resonators disposed at quarter-wavelength intervals along a dielectric stripline. The center or notch frequency of this filter is determined by the quarter-wavelength mechanical dimensions of each resonator in the filter.
Such filter structures of the prior art cannot be used effectively in a frequency hopping system, where it is required to rapidly and precisely change the desired frequency of the filter notch. This is because the notch frequency of such structures is fixed, i.e., determined by the quarter-wavelength mechanical dimensions of each resonator in the filter. These filters are also impractical for use in jamming applications because to protect a colocated high sensitivity receiver, a jamming amplifier must be provided with a dynamically-variable notch filter for rejecting jamming signals.
There is therefore a need to provide an improved high power tunable filter design having frequency characteristics which may be rapidly and precisely changed compatible with the requirements of a frequency hopping system. Such a filter would also be useful to protect a high sensitivity receiver from a deliberately colocated high power broadband jammer by rejecting the jammer output at dynamically-variable received frequencies.